1. Field of the Invention
The invention relates to a process for producing a barrel with a wear-resistant inner surface from a steel block for a twin-screw extruder with intermeshing screws, the process including the steps of producing two partially intersecting bores (i.e., forming a bore with a spectacle-shaped cross section) that pass all the way through the steel block, followed by hardening of the inner surface of the spectacle-shaped bore. The invention also relates to equipment for hardening the inner surface of the spectacle-shaped bore.
2. Description of the Prior Art
Twin-screw extruders with tightly intermeshing screws have barrels made of partial axial segments with a spectacle-shaped bore that passes completely through the barrel. The two extruder screws rotate in the spectacle-shaped bore a short distance away from its inner surface. The inner surface of the barrel bore is subject to abrasive wear, which has basically two causes. The first cause is the processing of feedstocks with aggressive properties, e.g., plastics that contain abrasive fillers. The second cause is related to the pressure conditions that arise inside the barrel during processing, which can lead to radial deflections of the screw shaft and thus to sliding contact between the outer surface of the screw and the inner surface of the barrel. For this reason, to the extent possible, barrels which are produced from a steel block are used in which the two partially intersecting bores for the two screws are produced, and the inner surface of the bores is subsequently hardened. This is usually accomplished by placing the machined workpiece in a hardening furnace, bringing it to hardening temperature, and then quenching it. This produces through-hardening of the workpiece, which is usually undesirable. For one thing, this applies to the flanges of the segments of the barrel, by which the segments are joined together to form a continuous barrel. For another, it applies to the two narrowed regions of the spectacle-shaped bore of the barrel, i.e., to the regions of the barrel near the intersecting zone of the two bores that pass through the barrel. Specifically, at these points it is desirable for the barrel to have the best possible toughness properties to avoid fractures, but toughness properties are generally lost as a result of through-hardening. In addition, the costs of this type of hardening treatment are relatively high, because the hardening treatment depends primarily on the weight of the workpieces, and a barrel segment for a twin-screw extruder is very heavy due to the thick walls that are required.
It is well known that the hardening of workpieces can be limited to the surface region of the given workpiece by case hardening, so that most of the material constituting the workpiece retains essentially its original good toughness properties. To do this, the workpiece, which is made of low-carbon steel, is placed in a furnace in which a carbon-rich atmosphere is maintained, so that the diffusion of carbon into the steel occurs to such an extent that subsequent cooling then produces the desired hardening effect. Nitriding of the surface region can be carried out in similar fashion by systematic diffusion of nitrogen into the steel. However, these types of processes are especially expensive due to the long annealing times that are required.
However, it is also basically well known that surface hardening can be carried out by heating an intrinsically hardenable steel only at the surface, i.e., to a limited depth of penetration, and then quenching it. The heating can be produced by flames or by inductive heating. In the latter case, electric eddy currents are induced by an inductor in the workpiece to be treated, which cause heating, especially due to the magnetic reversal of the workpiece.
Basically, inductive heating in constant form, i.e., with constant depth of penetration, can be carried out without any great problems in workpieces of uniform contour. For example, inductive heating is practiced especially successfully in the heating of steel pipes of circular cross section. Of course, in the case of barrels for twin-screw extruders, inductive heating by means of an inductor with a shape corresponding to the shape of the barrel cross section would lead to extremely nonuniform heating and depths of penetration. Specifically, the narrowed regions would be subjected to especially high thermal energy, since the energy input at those places would occur, as it were, from both barrel bores. Accordingly, the practical result upon cooling under hardening conditions would be through-hardening in the narrowed region. The result of this would be that these regions would become especially susceptible to fracture. Because of this excessive hardening, inductive hardening has never been used before.